1. Field of the Invention
The present invention relates to a high frequency switching circuit used for switching frequency bands and switching between transmission and reception signals in a radio communication apparatus, such as a cellular phone.
2. Description of the Related Art
High frequency switching circuits using high frequency switches composed of FETs for switching between transmission and reception or converting modes of wireless devices are widely used in battery-operated radio communication apparatuses, such as cellular phones.
In a high frequency switching circuit using FETs as high frequency switches, when a large-power transmission signal passes through the switching circuit, distortion occurs in the transmission signal, and the non-linear characteristic of the FET causes distortion of the transmission high-frequency signal which then causes harmonics.
FIG. 5 is a view illustrating a configuration of a high-frequency switching circuit according to the related art.
A high frequency switch 50 is connected between a transmission terminal 10 and an antenna terminal 30. A high frequency switch 60 is connected between a transmission terminal 20 and the antenna terminal 30.
Further, a high frequency switch 40 and a high frequency switch 70 may also be provided. The first frequency switch 40 has one terminal connected to the transmission terminal 10 and the other end connected to a ground. The high frequency switch 70 may have one end connected to the transmission terminal 20 and the other end connected to a ground.
An on/off control of each of the high frequency switches 40, 50, 60, and 70 is performed by using a voltage applied from a control circuit (not shown) that is provided at a switch control terminal thereof.
That is, when zero voltage is applied to each control terminal, the high frequency switch is turned off. When a voltage of approximately 2.5 to 4.2 V is applied thereto, the high frequency switch is turned on.
During the transmission, it is controlled so that the high frequency switch 50 and the high frequency switch 70 are turned on, and the high frequency switches 40 and 60 are turned off.
Here, in order to reduce distortion when the high-frequency switch generates a high output, a method of using a boost circuit 80, shown in FIG. 5, is known.
Here, a voltage of approximately 2.5 to 4.2 V of a control signal is boosted to a voltage of approximately 7 to 9 by using the boost circuit 80. The boosted voltage is applied to the control terminals of the high frequency switch 50 and the high frequency switch 70. A potential difference between gate•drain/source of each of the high frequency switches 40 and 60 in an off-state is set to be higher than a control signal voltage of approximately 2.5 to 4.2 V, thereby reducing the distortion of the high frequency switches 40 and 60 in the off-state.
FIG. 6 is a view illustrating a basic configuration of the high frequency switching circuit according to the related art. In FIG. 6, a single-pole, single-throw (SPST) switch is used as a high frequency switch.
That is, in the related art, the high frequency switching circuit including SPST switches suppresses distortion of the high frequency signal generated from the high frequency switch 40 in the off-state, that is, the high frequency switching circuit reduces distortion of the high frequency signal, such that when a high frequency signal A input from the transmission terminal 10 reaches the antenna terminal 30, a high frequency signal F that slightly produces small second and third harmonics 2F0 and 3F0 is obtained.
FIG. 7 is a view illustrating harmonic generation in the high frequency switching circuit according to the related art.
When the high frequency signal A that is applied to the transmission terminal 10 is transmitted to the antenna terminal 30, the harmonics generated due to the distortion caused by the high frequency switch 40 in the off-state can be reduced from a state E1 to a state E2 by boosting an output voltage of the boost circuit to a voltage of approximately 7 to 9 V.
Meanwhile, harmonics are generated by the high frequency signal A that is transmitted by the high frequency switch 50 in the on-state, which is shown as a state C in FIG. 7.
A harmonic E2 generated by the high frequency switch 40 in the off-state and a harmonic C generated by the high frequency switch 50 in the on-state are added to output a high frequency signal containing harmonics indicated by a sate F through the antenna terminal 30.
Even though the harmonics generated by the high frequency switch 40 in the off-state are reduced by using the method of reducing distortion according to the related art, a large distortion is caused by the high frequency switch 50 in the on-state, and harmonics generated by the distortion are added. As a result, the distortion is not significantly reduced.
As a method of reducing harmonics generated due to distortion caused by high frequency switches, a method disclosed in Japanese Patent Laid-Open Publication No. 2005-057375 is known.
The invention disclosed in Japanese Patent Laid-Open Publication No. 2005-057375, inserts phase adjusting lines and low-pass filters between the antenna terminal and the transmission terminal, and adjusts a phase difference between a progressive wave and a reflected wave such that the power of a composite wave made up of the progressive wave generated from the high frequency switch and the reflected wave resulting from reflection of the progressive wave from each of the LPFs.
Various kinds of methods of suppressing harmonic generation due to distortion caused by the high frequency switch have been proposed. However, the method of using the boost circuit increases power consumption of the boost circuit when a boost voltage is high.
Even when the boost circuit is provided, it may not result in a reduction in harmonics. Further, the method, disclosed in Japanese Patent Laid-Open Publication No. 2005-057375, requires devices, such as low-pass filters or phase adjusting lines. Therefore, it is difficult to reduce manufacturing costs or size of terminals, such as, cellular phones.